This invention relates to a novel method for the removal of water from aqueous alkali metal hydroxide solutions, and particularly to a method of removing water from aqueous sodium hydroxide solutions.
Alkali metal hydroxide solutions are commonly produced by electrolyzing an alkali metal halide solution in an electrolytic cell. Sodium hydroxide, for example, is commonly produced by electrolyzing sodium chloride brine solution in an electrolytic cell for producing chlorine. The cell usually generates about two molecules of hydroxide for each one molecule of chlorine. The sodium hydroxide, as an effluent stream of such cells, may be produced in various strengths, normally in concentrations of 8 to 40 percent by weight. In a diaphragm type chlorine cell, for example, sodium hydroxide may be present at 8 to 12 percent by weight, while production in a membrane type chlorine cell may result in sodium hydroxide concentrations of up to 40 percent by weight. However, for commercial use sodium hydroxide is generally concentrated to strengths of 50 percent by weight.
Concentration of the hydroxide solutions is normally accomplished by evaporating the water from the hydroxide stream using single or multiple effect evaporators. Energy requirements for this concentration step can total several thousand British Thermal Units (Btu's) per pound of hydroxide processed.
It is also known to concentrate sodium hydroxide or caustic solutions by treating the caustic solution with liquid ammonia as disclosed in U.S. Pat. Nos. 1,961,590 and 2,196,593. The liquid ammonia, in these processes, absorbs and removes the water in the caustic solution at a certain temperature and pressure forming a water-containing ammonia liquid phase which is subsequently removed from the caustic.
Although the aforementioned methods are operable, it is desired to provide a relatively simple method of removing water from an aqueous alkali metal hydroxide solution which is believed to be capable of being operated efficiently and economically